As an induction motor, a so-called squirrel-cage induction motor using a squirrel-cage rotor is known. This squirrel-cage induction motor is constituted by a stator in which a stator coil is disposed in a substantially cylindrical stator core having a plurality of stator slots and a rotor that is provided close to a radial inner side than the stator and is provided to be rotatable with respect to the stator.
The rotor includes a rotating shaft and a rotor core which is externally fitted onto and fixed to the rotating shaft. In the rotor core, a plurality of rotor teeth extending in a radial direction are radially disposed, and a rotor slot is formed between rotor teeth adjacent in a circumferential direction. A rotor bar (a conductor bar) is disposed in this rotor slot. An end ring (an end ring for short-circuiting) formed in an annular shape for surrounding the rotating shaft is provided at both ends in an axial direction of the rotor bar. The rotor bar and end ring constitute a secondary conductor.
Then, the rotor rotates by interaction between a magnetic field generated on the stator side and an induced current generated at the secondary conductor due to the magnetic field.
In recent years, efficiency regulation for induction motors (top-runner regulation) has been promoted. In addition, efficiency improvement has been promoted in terms of energy saving, low noise, further miniaturization, or the like even in induction motors used as main motors of railway cars, for example.
Here, when the miniaturization and high torque in induction motors are intended to be accomplished, it is necessary to increase a magnetic flux density in a gap formed between the stator core and the rotor core. However, spatial harmonic component is also largely increased, and consequently harmonic secondary copper loss (copper loss on the rotor side) also becomes large, and thereby there is possibility of degradation of motor characteristics.